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叚 supplies the sound — xiá unchanged.
叚 is the archaic form of
假 (to borrow); the meaning doesn't carry directly. Pure phonetic role. Same phonetic in
假 (borrow/false),
暇 (leisure — borrowed time),
瑕 (flaw in jade),
遐 (distant).
朝霞 'morning glow' and
晚霞 'evening glow' are the classic compounds; the character carries an unmistakably poetic register.